There are numerous known low-dispersion optical glasses. The SK-type glass listed in the “Glass Databook” published by the Japan Glass Product Industrial Association, commercial PCD4 glass made by HOYA Corporation, and the like are known glasses. However, these glasses generally have glass-transition temperatures of 560° C. or greater and are not suited for use in precision press molding. In a precision press molding glass, a high press molding temperature is problematic in that it damages the surface of the pressing mold and decreases the durability of the mold material. Thus, the glass-transition temperature of the glass is desirably as low as possible. To solve this problem, glasses containing large quantities of alkali metal oxides such as Li2O have been proposed.
The SiO2—B2O3—SrO—Li2O glass described in Japanese Patent No. 2872899 is one such example. However, since large amounts of Li2O are contained in the glasses described in the Patent, they have problems in the form of poor weatherability and in that the pressing mold employed during pressing tends to cloud. Japanese Patent No. 3150992 describes attempts to reduce the glass-transition temperature by incorporating alkalis and TeO2 into prior art SiO2—B2O3—BaO glass. However, the incorporation of alkalis and TeO2 results in substantial deterioration of chemical durability, rendering the glass unsuitable for high-precision molding.
The present invention, devised to solve the above-stated problems, has for its objects to provide a low-dispersion optical glass with a low glass-transition temperature and good weatherability; a press molding preform comprised of this optical glass; a method of manufacturing the same; an optical element comprised of this optical glass; and a method of manufacturing the same.